Where … at (Where was he at?) and where … to (Where is this leading to?) are often criticized as redundant because neither at nor to adds anything to the meaning of where, and sentences like the preceding ones are perfectly clear and standard without the final at or to. This criticism does not apply to where … from, which is fully standard: Where does the money come from? The constructions where … at and where … to occur in the speech of educated people but are rare in formal speech and edited writing.

(subordinating) in the place at which: where we live it's always raining

noun

4.

(usually pl) a question as to the position, direction, or destination of something

Usage note

It was formerly considered incorrect to use where as a substitute for in which after a noun which did not refer to a place or position, but this use has now become acceptable: we now have a situation where/in which no further action is needed

Word Origin

Old English hwǣr, hwār(a); related to Old Frisian hwēr, Old Saxon, Old High German hwār, Old Norse, Gothic hvar

at1

/æt/

preposition

1.

used to indicate location or position: are they at the table?, staying at a small hotel

2.

towards; in the direction of: looking at television, throwing stones at windows

3.

used to indicate position in time: come at three o'clock

4.

engaged in; in a state of (being): children at play, stand at ease, he is at his most charming today

5.

(in expressions concerned with habitual activity) during the passing of (esp in the phrase at night): he used to work at night

6.

for; in exchange for: it's selling at four pounds

7.

used to indicate the object of an emotion: angry at the driver, shocked at his behaviour

at

Lost in German and Dutch, which use their equivalent of to; in Scandinavian, however, to has been lost and at fills its place. In choosing between at church, in church, etc. at is properly distinguished from in or on by involving some practical connection; a worshipper is at church; a tourist is in the church.

The colloquial use of at after where ("where it's at") is attested from 1859. At last is recorded from late 13c.; adverbial phrase at least was in use by 1775. At in Middle English was used freely with prepositions (e.g. at after, which is in Shakespeare), but this has faded with the exception of at about, which was used in modern times by Trollope, Virginia Woolfe, D.H. Lawrence, and Evelyn Waugh, but nonetheless is regarded as a sign of incompetent writing by my copy editor bosses.

where

adv.

Old English hwær, hwar, from Proto-Germanic *khwar (cf. Old Saxon hwar, Old Norse hvar, Old Frisian hwer, Middle Dutch waer, Old High German hwar, German wo, Gothic hvar "where"), from PIE interrogative base *qwo- (see who).

where it's at

adverb phrase

At the site of stimulating and modish events, trends, etc; where the action is: Where the important stuff is going on. This is where it's at/ Why should only book writers write books? They're not where it's at/ TV is where it's at

noun phrase

The essential locus of the truth; the core of things: A lot of cats are finding out where it's at in the joint(1960+)

where it's at

Also,where the action is. The key center of activity; where important things are happening. For example, He decided to set up his store here, convinced that this is where it's at, or I'm going into the brokerage business; that's where the action is these days. The action or activity in this phrase can relate to just about anything—financial, political, social, or commercial.
[ ; c. 1960
]